This invention relates to drop marking devices, such as ink jet printers. Such devices are capable of marking a recording medium, such as paper, labels, and many other surfaces, by creating electrically charged ink drops which are directed onto the recording medium. Drops which are not charged or are inadequately charged do not reach the recording medium but instead pass into an ink catcher which returns the ink to a reservoir for further use. The ink drops are created by forcing the ink under pressure through a nozzle orifice to create a stream which is perturbated, for example, by a piezo-electric device associated therewith. The series of discrete drops thereby formed are then charged by a charging electrode, the degree of charge determining the amount of deflection when the drops thereafter pass through a constant electric field maintained by the deflection electrodes.
At the time that a drop is created its electrical charge must be established by the charging electrode. The charge to be placed on a drop is determined by the magnitude of the signal applied to the charging electrode. This signal is usually referred to in the art as the video signal. If the video signal is in the process of rising or falling or is not present at the time that a drop is formed the charge on the drop will not be proportional to the video signal as intended. This is usually referred to as a phase problem and must be overcome in order to reliably charge selected drops to accurately place drops on the recording medium.
In order to place specific charges on given drops it is necessary to know when drop separation is occurring, that is, the time relationship of drop formation relative to the video signal. If the video signal is not kept in phase with drop separation, the uniformity and fidelity of printing on the medium is adversely affected.
In order to maintain the correct phase relationship, ink jet systems are typically provided with a phase control system, usually of the feedback type, wherein low charge test drops, not intended to strike the recording medium, are generated, sent to the catcher, and their charge sensed. This information is used to alter the phase of the video signal to achieve the correct magnitude and phase of the video signal at the time that each newly formed drop passes through the charging electrode. The phase circuitry system itself forms no part of the present invention. A typical phase control circuit which can be used with the present invention is disclosed in U.S. Pat. No. 3,465,351, which patent is hereby incorporated by reference. Other phase control networks can be suitably employed with the present invention.
Phase control systems require the use of some type of sensing element for detecting the charge present on the test drops. One type of sensing element commonly employed is an ink catcher having a sensing electrode which contacts the test drops received in the catcher. The charge accumulation from the test drops produces a current flow, the magnitude of which, relative to the expected value, can be used to correct and maintain the phase relationship between the video signal and the formation of the drops.
To accurately determine the charge on the drops, it is necessary to provide a sensing electrode which is sensitive and accurate in a rather difficult environment. Specifically, it is necessary to detect very small charges from a collection of ink drops in an ink jet printing head remotely located from the phasing circuitry. Noise and spurious signals have heretofore required the use of shielding and fairly large collection passages to insure producing a reliable phasing signal. Such prior arrangements were bulky and, therefore, interferred with the desire to make the print head as small as possible. In one prior art device, manufactured by applicant's assignee, the ink catcher was provided with an electrically shielded return conduit of substantial length.
The object of the present invention is to provide an ink catcher with drop charge sensing means which is small in size and highly reliable.
It is a futher object to provide such an apparatus which has a high electrical impedance to obtain a significant reduction in signal noise.
A further object of the invention is to provide an ink catcher having an integral drop charge sensing means which avoids the bulk and assembly cost of prior art devices.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.